Electrochromic Nb-doped WO3 films: Effects of post annealing

Chun-Kai Wang a, Diptiranjan Sahu a,b,*, Sheng-Chang Wang c, Jow-Lay Huang a,d,e,**a Department of Materials Science and Engineering, National Cheng Kung University No. 1, Ta-Hsueh Road, Tainan City 701, Taiwan, ROC

b School of Physics, Materials Physics Research Institute and DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Private Bag 3,

Wits 2050, Johannesburg, South Africac Department of Mechanical Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwand Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan

e Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan

Received 21 October 2011; received in revised form 24 October 2011; accepted 21 November 2011

Available online 29 November 2011

Abstract

The Nb-doped WO3 films were deposited by e-beam co-evaporation method using ceramic WO3 targets and metal Nb slugs. The films were

analyzed by glancing incident angle X-ray diffraction (GIAXRD), UV/visible spectrophotometer, electrochemical cyclic voltammetry, X-ray

photoelectron spectroscopy (XPS). The as-prepared film is brown and amorphous in structure. The film has low transmission in optical visible

region. The XPS results indicate that the as-deposited film is non-stoichiometric. By applying a negative potential, the as-deposited film does not

show obvious electrochromic effect. However, the electrochromic properties of Nb-doped WO3 films are improved by post annealing treatment at

350, 400, and 450 8C in oxygen atmosphere. The Nb-doped WO3 films transform into crystalline structure and become transparent after post

annealing treatment. The energy band gap, optical modulation, and color efficiency increase with annealing temperature.

# 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Electrochromism; WO3; Annealing; E-beam evaporation

www.elsevier.com/locate/ceramint

Available online at www.sciencedirect.com

Ceramics International 38 (2012) 2829–2833

1. Introduction

The electrochromic materials possess the ability of

reversible and persistent change of optical properties by

double insertion/extraction of electrons and counter ions (H+,

Li+.) [1,2]. This unique property makes electrochromic

materials of great interest for application in different types

of optical devices, such as display, rear view mirror, and smart

windows [3–6]. Among diverse electrochromic materials,

tungsten oxide (WO3), which turns blue upon electrochemical

insertion and becomes transparent upon extraction, is by far the

most extensively studied materials prepared by vacuum

* Corresponding author at: School of Physics, Materials Physics Research

Institute and DST/NRF Centre of Excellence in Strong Materials, University of

the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa.

Tel.: +27 11 7176839/+886 6 2348188; fax: +27 11 7176879/+886 6 2763586.

** Corresponding author at: Department of Materials Science and Engineering,

National Cheng-Kung University No. 1, Ta-Hsueh Road, Tainan City 701,

Taiwan, ROC. Tel.: +27 11 7176839/+886 6 2348188;

fax: +27 11 7176879/+886 6 2763586.

E-mail addresses: Diptiranjan.Sahu@wits.ac.za (D. Sahu),

jlh888@mail.ncku.edu.tw (J.-L. Huang).

0272-8842/$36.00 # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserve

doi:10.1016/j.ceramint.2011.11.054

evaporation [7–9], chemical vapor deposition [10], hydro-

thermal [11] sol–gel [12,13], sputtering [14,15] and electro-

deposition [16]. For a few years, there is an extensive study for

the improvement of the properties of tungsten oxide by addition

of enhanced dopant [17–23]. Some compounds, such as TiO2,

MoO3 and Nb2O5, are used to increase coloration efficiency,

cycling lifetime, or reaction kinetics. Bathe and Patil [21]

shows that the cycle stability, charge storage capacity, and

reversibility can be improved by addition of Nb2O5 into WO3

films. Avellaneda’s study [24] indicates the mixed Nb2O5–WO3

films have larger optical modulation in photochromic reaction.

Rougier et al. [19] showed that the W–Nb–O films possess the

property of color neutrality which is promising for building

application in reduced state. However, there are few studies on

the effect of post annealing on electrochromic Nb-doped WO3

films. In this study, we prepared Nb-doped WO3 films by

electron evaporation method and investigated the influence of

post annealing treatment on the electrochromic properties.

2. Experimental procedure

The Nb-doped WO3 films are prepared by electron beam co-

evaporation method. The deposition targets are ceramic WO3

d.

Fig. 1. SEM images of (a) as-deposited and heat-treated films at (b) 350, (c) 400, (d) 450 8C in O2 atmosphere.

C.-K. Wang et al. / Ceramics International 38 (2012) 2829–28332830

bulks and metal Nb slugs. Before deposition, the vacuum

chamber is evacuated to 1.07 � 10�4 Pa and the deposition

pressure is controlled at 6.67 � 10�5–2.67 � 10�4 Pa. After

deposition, the Nb-doped WO3 films are post annealed in O2

atmosphere at 350, 400, and 450 8C for 2 h.

The film crystal structure was examined by Rigaku DMAX

2500 diffractometer with Cu Ka radiation of wavelength

0.1542 nm. The film surface morphology was studied by Philips

XL40 field emission scanning electron microscopy (FE-SEM).

The optical transmittance spectrum was measured by Hitachi U-

2001 UV/Visible Spectrophotometer in the range of 300–

1100 nm. The composition distribution profile and surface

chemical state were inspected by X-ray photoemission spectro-

scopy (PHI 5000 VersaProbe). The X-ray excitation source is Al

Ka radiation of energy 1486.6 eV. The carbon 1s peak with

binding energy 284.6 eVis used to calibrate tungsten and niobium

binding energy. The cycling voltammogram tests were carried out

on VersaStat II Electrochemical Workstation with three electrodes

configuration. The counter and reference electrodes were

platinum and saturated calomel electrode (SCE). The electrolyte

was 0.1 M LiClO4/propylene carbonate (PC) solution.

3. Results and discussion

The thickness of the Nb-doped WO3 films deposited by

electron beam co-evaporation is about 350 nm. The as-

deposited film contains 27.79% W, 17.30% Nb, and 53.1%

O and the composition is well distribution, as examined by

XPS. Fig. 1 shows the morphology of Nb-doped WO3 films for

as-deposited state and the post annealed films at 350, 400,

450 8C in O2 atmosphere. It is observed that grain size increases

with annealing temperature.

Fig. 2 shows the GIAXRD patterns of Nb-doped WO3 films

at three different postannealing temperatures in O2 atmosphere.

The GIAXRD pattern of as-prepared film is amorphous

structure without any obvious diffraction peaks. After post

annealing process, a diffraction peak at 258 appears and the

peak intensity increases with post annealing temperature. It can

be explained by the fact that the electrochromic films have

crystallized by post annealing treatment. Compared to the

Bathe’s results [21], the GIAXRD patterns do not reveal the

formation of solid solution WNb2O8 even after annealing at

450 8C in O2 atmosphere. This difference may be caused due to

different preparation methods. Bathe prepared W–Nb–O films

by sol–gel method in which the chemical activity of precursor is

higher than that of atomic evaporation method. The as-

deposited Nb-doped WO3 film is deep brown color and the

optical transmittance is below 20%. The optical transmittance

of Nb-doping WO3 film is increased by post annealing process

in O2 atmosphere, as shown in Fig. 3. The optical transmittance

of Nb-doped WO3 increase gradually from as-deposited state to

400 8C post-annealed film and the optical transmittance of Nb-

doping WO3 film does not increase further even after post

annealing at 450 8C. The film color becomes transparent after

80706050403020

WO3

* *

450 oC

400 oC

Inte

nsity

(arb

. uni

ts)

2θ (degree)

as-deposited

350 oC

*ITO substrate

Fig. 2. The GIAXRD patterns of Nb-doping WO3 film after three different

postannealing temperatures in O2 atmosphere.

110010009008007006005004003000

10

20

30

40

50

60

70

80

90

100

Tran

smitt

ance

(%)

Wavelength (nm)

as-deposited 350 oC 400 oC 450 oC

Fig. 3. The optical transmittance spectra of Nb-doped WO3 films at as-

deposited state and after post annealing treatment at 350, 400, and 450 8Cin O2 atmosphere.

C.-K. Wang et al. / Ceramics International 38 (2012) 2829–2833 2831

heat treatment at 400 and 450 8C in O2 atmosphere. The band

gap energy can be calculated from Tauc’s law [25] using the

relation of (ahv)n = A(hv � Eg), where a is the absorption

coefficient, n is a characteristic constant depends on the

material, A is a constant, and Eg is the band gap energy of the

film. For the indirect allowed transition Nb-doped WO3 films, n

is equal to 2. The band gap energy of Nb-doped WO3 films are

1.98, 3.58, 3.69, and 3.69 eV for as-deposited state and

annealed films of 350, 400, and 450 8C, respectively. The

42403836343230

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

experimental data fitting peaks sum background W+6 4f7/2

W+6 4f5/2

W+4 4f7/2

W+4 4f5/2

at as-deposit ed stateW 4f7/2&5/2

Binding Energy (eV) 42403836343230

after 450 oC heat-treatmentW 4f7/2&5/2

experimental data fitting peaks sum background W+6 4f7/2

W+6 4f5/2Inte

nsity

(arb

. uni

ts)

Fig. 4. The XPS spectra of W 4f7/2 and 5/2 and Nb 3d5/2 and 3/2 of Nb-doped W

variation of band gap energy could be caused by the increase of

chemical stoichiometry and crystal growth effect. The chemical

states of W and Nb atoms of as-deposited and annealed film at

450 8C are examined by XPS and the results shown in Fig. 4.

The W4f XPS spectra contain three major peaks at 33.2, 35.4

and 37.6 eV. Those peaks can be decomposed into two doublets

of W+6 (35.6, 37.7 eV) and W+4 (32.9, 35.1 eV) ions of W 4f7/2

and W4f5/2 [26]. The XPS spectra of Nb atom for as-deposited

state show two major peaks at 207.2 and 209.8 eV and a small

217215213211209207205203201

Inte

nsity

(arb

. uni

ts)

Binding Energy (eV)

Binding Energy (eV)

experimental data fitting peaks sum background Nb+5 3d5/2

Nb+5 3d3/2

Nb+2 3d5/2

Nb+2 3d3/2

at as-deposited stateNb 3d5/2&3/2

217215213211209207205203201

at af ter 450 oC heat treatentNb 3d5/2&3/2

experimental data fitting peaks sum background Nb+5 3d5/2

Nb+5 3d3/2

Inte

nsity

(arb

. uni

ts)

O3 films at as-deposited and after heat treatment at 450 8C in O2 atmosphere.

1.00.50.0-0.5-1.0-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

as deposited state 450 oC heat-treated film

Cur

rent

Den

sity

(mA

/cm

2 )

Potential vs. SCE (volts)

Fig. 6. The cycle voltammogram of electrochromic films for as-deposited state

and 450 8C heat-treated of Nb-doped WO3 films.

110010009008007006005004003000

10

20

30

40

50

60

70

80

90

100

Tran

smitt

ance

(%)

Wavelength (nm)

as-depsotied bleach as-depsotied color 450 oC bleach 450 oC color

Fig. 5. The optical transmittance spectra of Nb-doped WO3 film at as-deposited

state and at heat-treated at 450 8C after bleach and color reaction.

C.-K. Wang et al. / Ceramics International 38 (2012) 2829–28332832

peak at 204.6 eV. Those XPS peaks are the superimposition of

the doublets of Nb+5 (207.4, 209.7 eV) and Nb+2 (204.4,

206.3 eV) ions for Nb 3d5/2 and Nb 3d3/2 [27]. From those

results, we can conclude that the chemical compositions in as-

deposited Nb-doped WO3 film are WO2, WO3, Nb2O5 and

small amount of NbO. It can be inferred that the Nb atom reacts

with oxygen of WO3 to form NbO and Nb2O5. On the other

hand, as XPS analysis was performed from the surface of 5 nm

depth film, the exposure of Nb-doped film in air may be another

reason for existence of large amount of Nb2O5. After annealing

treatment at 450 8C in O2 atmosphere the XPS spectra shows a

W4f7/2 and 5/2 doublet at 35.4 and 37.6 eV and a doublet Nb

3d5/2 and 3/2 at 207.2 and 209.8 eV, which is stoichiometric

chemical composition of WO3 and Nb2O5. The as-deposited

film and annealed film at 450 8C are colored at �1 V in LiClO4/

PC solution for 60 s.

The optical transmittance spectra of the films are shown in

Fig. 5. In the coloration reaction, the W+6 ions are reduced into

W+5 and the Nb+4 are reduced into Nb+4 to bring out the

electrochromic effect. For the as-deposited film, the optical

transmittance does not vary obviously before and after

electrochemical insertion of Li+ ions and the film color is

504540353025

W 4f7/2&5/2

after one insertion/extracion cycle

as-deposited state

Inte

nsity

(arb

uni

ts)

Binding Energy (eV)

Fig. 7. The XPS spectra of W4f and Nb 3d of Nb-doped WO3 films fo

still brown. The color of 450 8C annealed film becomes blue

after Li+ intercalation reaction at �1 V. The variation of optical

transmittance at bleach and color state of as-deposited and 350,

400, and 450 8C annealed films are 0.6, 19, 35, and 38.4% at

wavelength of 633 nm. The coloration efficiency of the films is

calculated using the equation of CE = DO.D./DQ. The DO.D is

the change of optical density and is equal to log(Tb/Tc), where

Tb and Tc are the optical transmittance at bleach and color state

at specific optical wavelength. DQ is the reacted charge density

in the color or bleach reaction. The coloration efficiencies at

633 nm are 4, 18.5, 22, and 23 C/cm2, respective for as-

deposited film and 350, 400, and 450 8C annealed films. The

increase of color efficiency with annealing temperature is due

to chemical stoichiometry of electrochromic films.

Cyclic voltammetry (CV) were measured for as-deposited

and 450 8C heat-treated films at the potential range of �1 to 1 V

in three electrodes configuration with scan speed of 50 mV/s

and the results are shown in Fig. 6. The CV loop of as-deposited

film has a larger area compared to that of 450 8C annealed film

and there are two pairs of cathodic/anodic current peaks. On the

other hand, the 450 8C annealed film has only one pair of

cathodic/anodic current peaks. However, in the Bathe’s study,

220215210205200195

Nb 3d5/2&3/2

Inte

nsity

(arb

uni

ts)

Binding Energy (eV)

as-deposited state

after one insertion/extracion cycle

r as-deposited state and that after one insertion/extraction reaction.

C.-K. Wang et al. / Ceramics International 38 (2012) 2829–2833 2833

they show only one pair of cathodic/anodic peak for 2% Nb–

WO3 films. In order to clarify the electrochemical mechanism

in the insertion/extraction reaction, we compare the XPS

spectra for as-deposited state and that after insertion/extraction

cycle, which are shown in Fig. 7. It clearly shows that the peak

intensity of W+4 and Nb+2 decrease after one insertion/

extraction cycle. We can reasonably infer that those two pairs of

cathodic/anodic peaks are consisted of the oxidation of WO2

and NbO into WO3 and Nb2O5.

4. Conclusion

The Nb-doping WO3 electrochromic films are prepared

successfully by electron beam method, where tungsten and

niobium atoms are well distributed. The as-deposited film is

amorphous and non-stoichiometric. The film does not show

obvious electrochromic effect and in CV cycling the WO2 and

NbO are oxidized into WO3 and Nb2O5. After post annealing in

O2 atmosphere, the film transforms into crystalline structure and

become chemically stoichiometric. The energy band gap and

electrochromic effect also increase with annealing temperature.

Acknowledgements

This work was supported by the National Science Council of

Taiwan under contract of NSC96-2218-E-006-006. Author

D.R. Sahu is also thankful to National Research Foundation,

South Africa for supporting this work.

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